The present invention relates to an apparatus for separating the liquid and vapour phases of a fluid and the application of such an apparatus to the reduction of the moisture content of steam leaving a steam generator.
Generally, a certain quantity of a fluid in the vapour phases is in the liquid phase. For numerous applications, the permitted percentage of a fluid in the liquid phase is well below the existing percentage and it is then necessary to pass the fluid through a suitable separating apparatus permitting the passage of the fluid in the vapour phase, whilst retaining most of the said fluid in the liquid phase transported by the vapour.
Thus, for example, steam generators in which a primary heat transfer fluid heats the water circulating in a secondary circuit to bring the latter to the vapour state generally produce saturated steam having a relatively high moisture content. This is more particularly the case when the primary fluid circulates in U tubes immersed in the water within the generator. In this type of steam generator, the moisture content of the steam produced varies between 20 and 80% by weight, as a function of the operation and utilization conditions. This moisture content is not compatible with the maximum moisture content permitted in the turbines generally driven by the steam leaving the steam generators. Thus, the steam driving the turbines must contain no more than 0.25% of liquid. It is therefore necessary to bring about an optimum complete separation of the liquid and vapour phases before the steam reaches the turbines. This separation is generally carried out within the steam turbines and specifically within the upper dome thereof.
Already, numerous separation apparatuses are known, which are positioned in the upper dome of steam generators. These known apparatuses generally operate according to the cyclone principle, by gravity or by means of labyrinth seals and retaining channels. However, they all have the disadvantage of either a relatively low efficiency level involving the arrangement in series of a large number of said separation apparatuses in the upper dome of the generator, or of having a high pressure drop which reduces the recirculation rate, or have overall dimensions such that they occupy a large area within the generator. In all three cases, the total overall dimensions of the separation apparatus or apparatuses within the steam generator are very great and leads to a significant increase in the total height of the generator compared with that occupied by the actual heat exchanger.